The present invention relates to a honeycomb structure. More specifically, the present invention relates to a honeycomb structure capable of inhibiting crack generation of the outer peripheral coat layer and/or deterioration in bonding strength of the bonding layer containing biologically soluble fibers can be inhibited, and a honeycomb structure capable of inhibiting crack generation of the outer peripheral coat layer.
In order to trap particulate matter such as dust contained in automobile exhaust gas or incineration exhaust gas generating upon incineration of waste, a honeycomb filter of a ceramic honeycomb structure is used. In particular, in order to efficiently remove particulate matter (hereinbelow sometimes referred to as “PM”) such as soot discharged from an internal combustion engine, a diesel particulate filter (hereinbelow sometimes referred to as a “DPF”) is used.
In the DPF, since clogging is finally caused if the trapped PM is not removed, it is necessary to regenerate the DPF by removing the trapped PM. The DPF can be regenerated generally by combusting PM by heating the DPF itself. As a DPF regeneration method, there is, for example, a method where an oxidation type catalyst is loaded on the DPF to efficiently combust PM by high-temperature exhaust gas generating in a diesel engine. However, in the case of regenerating a DPF by heating, temperature distribution in the honeycomb structure becomes uneven by rapid temperature change of exhaust gas or local heat generation, and there arises a problem of crack generation or the like. To solve the problem, there has been disclosed a honeycomb structure of a bonded article where a plurality of honeycomb segments are bonded together at an outer wall of each of them (see, e.g., U.S. Pat. No. 4,335,783).
In addition, a honeycomb structure having such structural characteristics generally has a problem of low mechanical strength because of thin partition walls and high porosity. Therefore, mechanical strength is improved particularly in a large-sized honeycomb structure. For example, there has been disclosed a honeycomb structure, where the outer peripheral portion of the bonded article obtained by bonding a plurality of honeycomb segments at an outer periphery of each of them with a bonding material is subjected to grinding to arrange an outer peripheral coat layer of an outer peripheral coat material on the outer peripheral face (see, e.g., JP-B-51-44713).
Such a bonding material and an outer peripheral coat material generally contain inorganic particles such as a cordierite powder and silicon carbide particles and an inorganic binder such as silica sol and alumina sol. There is a case of further containing ceramic fibers as necessary. In particular, in recent years, there has been disclosed a honeycomb structure using a bonding material or an outer peripheral coat material containing biologically soluble fibers in consideration of influences on the human body (see, e.g., JP-A-2003-105662).
However, in a bonding material or an outer peripheral coat material containing biologically soluble fibers, there is a case that characteristics such as flowability are changed with the passage of time due to a metal ion extricated from the biologically soluble fibers and the resultant rise in pH. When a honeycomb structure is manufactured by the use of such a bonding material or an outer peripheral coat material having changed characteristics, bonding strength in a bonded portion may be deteriorated, or a crack may be generated in the outer peripheral coat layer.